Pre-coating the leads or pins of a metal frame is a time consuming technique, especially with increasingly small sizes and increased densities of the pins. The leads or pins are commonly die stamped copper or Fe/Ni alloy for soldering onto a printed circuit board with an eutectic Sn/Pb alloy.
Recent developments have provided alternative techniques based on the use of assembling die stamped metal frames pre-coated with a layer or a multilayer of metals or alloys exhibiting a high compatibility with silicon dice assembling operations. Compatibility is also exhibited with the soldering of the connecting wires, as well as with the eventual soldering of the tips of the pins of the device on respective metal pads. The metal pads are defined on the printed circuit board. The soldering is a conventional soldering technique using an eutectic Sn/Pb alloy.
The use of die stamped frames of copper or of any other adequate metal are becoming more frequent. These frames are pre-coated with a nickel and/or a nickel-palladium and/or a gold film. The coatings, whose thickness may range between 1 and 2 .mu.m, are very effective in promoting an adequate wettability of the coated pin surfaces by the molten eutectic Sn/Pb alloy used in assembling the devices on the printed circuit board. Manufacturers of integrated devices have the opportunity of eliminating from the fabrication process the steps of pre-coating the pins with eutectic alloy, or any other plating treatment of the metal frames. These items may be purchased from specialized suppliers already pre-coated with the above noted metal films chemically or galvanically deposited onto the entire die-stamped frame.
On the other hand, conventional die stamping of a copper strip produces a modular patterning of several frames in special stamping molds. The various functional features or portions of each frame remain connected to each other and to the surrounding metal, or to a second line of connecting metal bridges referred to as a connection bar. They will be separated during the final steps of the fabrication process of the integrated devices. Only after completing the encapsulation of the device, the single pins are separated from the surrounding metal or from the connection bar. Also, the pins themselves are separated by cutting the metal bridge connections along the so-called perimetral dam bar line, upon which abut the edges of the mold used for encapsulating with resin the integrated device during the assembling process. The pins are also separated by a perimetral trimming performed internally of the outermost perimeter of the die stamped area of each frame during the patterning thereof, before or after bending the pins. The array of adjacent pins are separated, trimmed and bent into shape using dedicated molds, or by the use of special bending devices. The bending devices include interpin cutters and tip trimming devices. As a result, the pins have a precise shape to superficially mount them on a printed circuit board.
FIGS. 1a and 1b show metal frames defined by die stamping a copper strip, as well as details of the patterning of the pins. A broken line indicates the perimeter of the cut performed after encapsulation, and the dash and dot lines indicate the cuttings that are performed to break the metal bridge connections along the so-called dam bar. FIGS. 2a, 2b and 2c illustrate forming operations of the pins carried out by rolling, as well as the final cutting and trimming of the tips of the pins. FIGS. 3a and 3b show an alternative technique of forming and trimming the pins by using cam mechanisms.
The shaping, cutting and trimming techniques of the pins of the encapsulated device always involve a cutting of the outer end or tip of the pins. For conventional pre-tinned pins, as shown in FIG. 4, soldering to the pad of the printed circuit normally has an optimal web configuration. In the case of thin film precoated metal frames, for example, with a nickel-palladium alloy or with gold, the soldering takes a more critical configuration, as depicted in FIG. 5.
The frames plated with thin metal films are extremely thin and the wettability of the cut end surface of the pins by the molten eutectic Sn/Pb alloy during the soldering of the surface mount devices on the printed circuit board is often inadequate to reliably establish a good electrical and mechanical connection of the pin to the respective metal pad. The thin metal films typically include nickel, nickel-palladium, gold and similar metals or alloys. This is especially so for the most external or top coating film, which usually contains a noble metal, such as palladium or gold, and may or may not be alloyed to other metals.
The reasons for such a different performance of pre-tinned frames from that of pre-plated frames with a noble metal containing a thin top coating, even though both are subjected to trimming, are not clear. For instance, when trimming a copper frame pre-coated with a rather thick layer of tin, the cut end surface of the pins remains sufficiently wettable by the eutectic Sn/Pb alloy during the soldering phase of the devices onto the board. This is by virtue of a significant smearing of the tin over the cut surface by the cutting blade.
In contrast, pre-coating the frames with nickel-palladium, gold, and similar metal films containing a noble metal, with a thickness marginally smaller than that of the tin layer of a tinned frame, does not provide a significant transfer of noble metal on the cut face of the pins. Therefore, the wettability of the bare copper of the trimmed end of the pins is inadequate to ensure an optimal shape of the web of solder.